Continuously variable hydrostatic transmission including a pulse width modulation ratio controller

Abstract

A controller for changing the ratio of a continuously variable hydrostatic transmission, as determined by the angular position of a swashplate, including an actuator having a cylinder slidingly receiving a piston to define a pair of chambers. A pair of solenoid valves, each configured to connect a respective chamber selectively to sump pump pressure or atmospheric pressure, are energized with a stream of pulses to create differential fluid pressures in the chambers, thereby producing movement of the piston; the piston movement being linked to the swashplate to produce a corresponding adjustment of the swashplate angular position. The rate of change of the swashplate angular position is controlled by pulse width modulating the pulse stream.

What is claimed is: 
     
       1. A ratio controller for a continuously variable hydrostatic transmission including a hydraulic pump unit, a hydraulic motor unit, and a swash plate positioned between the hydraulic pump and motor units to accommodate flows of hydraulic fluid between the hydraulic pump and motor units, the ratio controller comprising, in combination: an actuator including a cylinder and a piston disposed in the cylinder to define first and second chambers;   a first fluid valve having a quiescent valve position connecting the first chamber to a source of hydraulic fluid pressure and an actuated valve position venting the first chamber;   a second fluid valve having a quiescent valve position connecting the second chamber to the hydraulic fluid pressure source and an actuated valve position venting the second chamber;   a module responsive to command signals for controllably activating a selected one of the first and second fluid valves with a stream of electrical pulses to shift the selected fluid valve to its actuated position for the duration of each pulse, the selected fluid valve shifting back to its quiescent valve position for the duration of each interval between pulses, thereby creating differential fluid pressures in the first and second chambers to produce relative motion of the actuator piston and cylinder; and   means for translating the relative motion of the actuator piston and cylinder into transmission ratio-changing movement of the swashplate.   
     
     
       2. The ratio controller defined in claim 1, wherein the module modulates one of the frequency and the width of the pulse stream to control a rate of transmission ratio-changing movement of the swashplate. 
     
     
       3. The ratio controller defined in claim 2, wherein the first and second fluid valves are solenoid valves, and the pulse stream is a current pulse stream. 
     
     
       4. The ratio controller defined in claim 3, wherein the module pulse width modulates the pulse stream. 
     
     
       5. The ratio controller of claim 1, further comprising fluid flow restrictors included in fluid lines connecting the first and second chambers to the hydraulic fluid pressure source and in fluid lines venting the first and second chambers. 
     
     
       6. A method for controlling the ratio of a continuously variable hydrostatic transmission including a swashplate operatively positioned between a hydraulic pump unit and a hydraulic motor unit, the method comprising the steps of: linking an actuator to the swashplate, the actuator including a piston disposed in a cylinder to define the first and second chambers;   providing a source of pressurized hydraulic fluid;   providing a first solenoid fluid valve having an actuated valve position venting the first chamber and a quiescent valve position connecting the first chamber to the pressurized hydraulic fluid source;   providing a second solenoid fluid valve having an actuated valve position venting the second chamber and a quiescent valve position connecting the second chamber to the pressurized hydraulic fluid source;   setting the transmission ratio by maintaining the first and second solenoid valves in their quiescent valve positions to equalize fluid pressures in the first and second chambers and maintain a ratio-setting position of the swashplate;   changing the transmission ratio by energizing one of the first and second solenoid valves with a stream of electrical pulses to shift the one solenoid valve to its respective actuated valve position for the duration of each electrical pulse and to shift the one solenoid valve to its respective quiescent valve position for the interval between each electrical pulse, while maintaining the other of the first and second solenoid valves in its respective quiescent valve position, thereby creating differential fluid pressures in the first and second chambers to produce relative movement of the actuator cylinder and actuator piston; and   translating the relative movement of the actuator cylinder and actuator piston into ratio-changing motion of the swashplate.   
     
     
       7. The method defined in claim 6, wherein the energizing step includes the step of modulating the pulse width of the pulse stream to vary the rate of ratio-changing motion of the swashplate. 
     
     
       8. The method defined in claim 7, further including the steps of terminating the pulse stream when the swashplate achieves a commanded transmission ratio-setting position. 
     
     
       9. A continuously variable hydrostatic transmission comprising: a housing;   an input shaft journaled in the housing for receiving input torque from a prime mover;   an output shaft journaled in the housing for imparting output torque to a load;   a hydraulic pump unit driven by the input shaft;   a hydraulic motor unit grounded to the housing;   a wedge-shaped swashplate pivotally connected to the output shaft in torque coupled relation, the swashplate operatively positioned between the hydraulic pump and motor units and having ports accommodating transfer of hydraulic fluid between the hydraulic pump and motor units; and angular position of the swashplate relative to an axis of the output shaft determining a transmission ratio; and   a controller including: an actuator including a piston disposed in a cylinder to define first and second chambers,   a first fluid valve having a quiescent valve position connecting the first chamber to a source of hydraulic fluid pressure and an actuated valve position venting the first chamber;   a second fluid valve having a quiescent valve position connecting the second chamber to the hydraulic fluid pressure source and an actuated valve position venting the second chamber;   a module responsive to command signals for controllably actuating a selected one of the first and second fluid valves with a stream of electrical pulses to shift the selected valve to its actuated position for the duration of each pulse, the selected fluid valve shifting back to its respective quiescent valve position for each interval between electrical pulses, thereby creating differential fluid pressures in the first and second chambers to produce relative motion of the actuator piston and cylinder; and   means translating the relative motion of the actuator piston and cylinder into transmission ratio-changing angular movement of the swashplate.     
     
     
       10. The ratio controller defined in claim 9, wherein the module modulates one of the frequency and the width of the pulse stream to control a rate of transmission ratio-changing movement of the swashplate. 
     
     
       11. The ratio controller defined in claim 10, wherein the first and second fluid valves are solenoid valves, and the pulse stream is a current pulse stream. 
     
     
       12. The ratio controller defined in claim 11, wherein the module pulse width modulates the pulse stream. 
     
     
       13. The ratio controller of claim 12, further comprising fluid flow restrictors included in fluid lines connecting the first and second chambers to the hydraulic fluid pressure source and in fluid lines venting the first and second chambers.